A method and apparatus for controlling the thickness of a semiconductor wafer during a backside grinding process are disclosed in U.S. Pat. No. 6,368,881B1. Optical measurements of the wafer thickness during a backside grinding process are used to determine the endpoint of the grinding process. Furthermore, the method may be used to determine if wedging of the semiconductor occurs and, if wedging does occur, to provide leveling information to the thinning apparatus such that a grinding surface can be adjusted to reduce or eliminate wedging.
However, it is believed that this method and apparatus are not suitable to reliably measure wafer thicknesses for silicon wafers with a highly-doped silicon layer on the backside of the silicon wafer due to a low-contrast interference signal.
It is therefore desirable to provide apparatus and a method for monitoring a thickness of a silicon wafer with a highly-doped layer at least at a backside of the silicon wafer, which provide a sufficiently high-contrast interference signal.